ASASSN-16ae: A Powerful White-Light Flare on an Early-L Dwarf

Abstract

We report the discovery and classification of SDSS~J053341.43+001434.1 (SDSS0533), an early-L dwarf first discovered during a powerful $\Delta V<-11$ magnitude flare observed as part of the ASAS-SN survey. Optical and infrared spectroscopy indicate a spectral type of L0 with strong H$\alpha$ emission and a blue NIR spectral slope. Combining the photometric distance, proper motion, and radial velocity of SDSS0533 yields three-dimensional velocities of $(U,V,W)=(14\pm 13,-35\pm 14,-94\pm 22)$~km~s${}^{-1}$, indicating that it is most likely part of the thick disk population and probably old. The three detections of SDSS0533 obtained during the flare are consistent with a total $V$-band flare energy of at least $4.9\times 10^{33}$~ergs (corresponding to a total thermal energy of at least $E_{\mathrm{tot}}>3.7\times 10^{34}$~erg), placing it among the strongest detected M dwarf flares. The presence of this powerful flare on an old L0 dwarf may indicate that stellar-type magnetic activity persists down to the end of the main sequence and on older ML transition dwarfs.

7 pages, 3 tables, 2 figures; accepted to ApJ Letters; updated to reflect referee response and proof corrections

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